3.3

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annabel jones

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Plants need water for:
Photosynthesis – to create glucose
Structure/support – vacuole presses cytoplasm against cell walls  turgidity
Transport of dissolved nutrients
Cooling
Waxy cuticle has to help prevent water loss
The upper epidermis is thin and transparent to allow more light through to the chloroplasts
Palisade cells are full of green chloroplasts, containing chlorophyll to trap sunlight
Spongy mesophyll contains lots of air spaces to allow carbon dioxide to diffuse through the leaf and increase the surface area.
Stomata are holes in the leaf to allow carbon dioxide to diffuse in and oxygen to diffuse out
Guard cells open and close the holes to prevent water loss
Size and exchange in plants
Plants continue to grow throughout their lives.
Although some plants are small, many perennial plants (plants that live a long time and reproduce year after year) are large.
This means plants need very effective transport systems to move substances both up and down from the top of the roots to the topmost leaves and stems.
SA:V ratio and exchange in plants
SA:V ratios are not simple in plants.
Leaves are adapted to have a large SA:V ratio for gas exchange with the air.
However, the size and complexity of multicellular plants means that when the stems, trunks, and roots are taken into account, they still have a relatively small SA:V ratio.
This means they cannot rely on diffusion alone to supply their cells with everything they need.
Metabolic demands and exchange in plants
Non-green parts of plant (e.g. roots) can’t photosynthesis.
They need glucose and oxygen (for respiration) transported to them.
They need waste products of metabolism removed.
Hormones made in one part of a plant need transporting to their target tissue.
Mineral ions absorbed by the roots need to be transported to all cells to make the proteins required for enzymes and the structure of the cell.
Plants with specialised transport systems are vascular plants.
Plants exchange and transport:
Carbon dioxide – for photosynthesis
Oxygen – for respiration
Water – for photosynthesis, structure/support, transport of dissolved nutrients, cooling
Organic nutrients (e.g. sugars, starch) – for respiration, storage
Inorganic ions (e.g. nitrate, phosphate, potassium) – for healthy growth, making proteins, making chlorophyll, etc
Cotyledons = organs that act as food stores for a developing embryo plant, and form the first leaves when the seed germinates.
Dicots (dicotyledonous plants) = plants that make seeds containing two cotyledons.
Herbaceous dicots = dicots with soft tissues and a relatively short lifecycle
Transpiration
Water and mineral ions
Xylem vessels
Up only
Passive process
Translocation
Sugars & amino acids (assimilates)
Phloem
Bidirectional
Active process
Dicots have vascular tissue distributed throughout the plant.
The xylem and phloem are found together in vascular bundles.
Xylem – dead tissue (transports water and minerals)
Phloem – sieve tube elements and companion cells (transports assimilates such as sugars)
In roots and stem, the xylem tissue is found on the inside.
However, in leaves, xylem is found above phloem tissue.
Vascular bundle in leaves
The vascular bundles form the midrib and veins of a leaf.
A dicot leaf has a branching network of veins that get smaller as they spread away from the midrib.
Within each vein, the xylem is located on top of the phloem
In between the xylem and phloem is a layer of cambium.
Vascular bundle in roots
The vascular bundle is in the centre of a young root.
There is a central core of xylem, often in the shape of an X
The phloem in between the arms of the X
This arrangement provides strength to withstand the pulling forces to which roots are exposed.
Around the vascular bundle is a special sheath of cells called the endodermis.
This has a key role in getting water into the xylem vessels. Just inside the endodermis is a layer of meristem cells called the pericycle.
Vascular bundles also contain other types of tissue to give the bundle some strength and help to support the plant:
Sclerenchyma
Collenchyma
Parenchyma
Sclerenchyma
Support and structure (around vessels).
Thickened with lignin (so cells are dead) and cellulose (more than in typical cells).
They strengthen stems and leaf midribs.
Fibres are one type of sclerenchyma cell.
Collenchyma
Found by epidermis and involved in growth.
They have thick cellulose walls, giving strength to vascular bundles and outer parts of stems.
Flexible support providing wind resistance.
Parenchyma
A soft packing tissue in plants which fills spaces between other tissues.
In roots, parenchyma cells may store starch.
In leaves, some have chloroplasts and can photosynthesise.
In aquatic plants, parenchyma has air spaces to keep the plant buoyant.
Xylem's main functions are support and water/mineral transport.
The flow of materials in the xylem is up from the roots to the shoots and leaves.
The xylem also help to structurally support the plant.
Xylem is made up of several types of cells, most of which are dead when they are functioning in the plant:
Vessels
Parenchyma
Fibres
The xylem vessels are made by several columns of cells fusing together end to end.
Water and mineral ions are transported are transported through this tube.
Adaptations of xylem vessels include:
The contents of the cells decay, so the cells do not contain any cytoplasm or organelles (as these would slow down the flow of water). This leaves a long, hollow tube.
The end plates break down, allowing unimpeded flow of water.
The cell walls are thicker than usual, to withstand the pressure of the water flowing through.
Cell walls are lignified in patterns (spiral, rings, or broken rings).
Lignin kills the cells, but:
Makes the cells waterproof
Adds strength to withstand the pressure of the moving water
Keeps the vessels open at all times
The patterns of deposition allow some flexibility
Bordered pits = small non-lignified regions of the xylem vessel walls.
They allow water and mineral ions to move laterally (sideways) between xylem vessels.
Tannin = a bitter-tasting chemical that protects plant tissues from attack by herbivores, and infection from bacteria and fungi.
Transpiration = the loss of water from the stomata in the leaves as a result of evaporation.
The main function of the phloem tissue is translocation.
This is the transport of assimilates (organic compounds, particularly sucrose) from sources (e.g. leaves) to sinks (e.g. roots).
These assimilates are dissolved in water to form sap.
They are transported up and down the plant.
Phloem tissue mostly consists of sieve tube elements and companion cells.
Other cell types include parenchyma for storage, and strengthening fibres.
Mature phloem tissue contains living cells, unlike xylem tissue.
Sieve tube elements line up end to end.
They are alive.
Their function is to transport assimilates around the plant.
Adaptations of sieve tubes include:
Elongated
No nucleus
Very little cytoplasm
Tonoplast (vacuole membrane) and other organelles break down.
Unlike in xylem vessels, the sieve tube's end walls do not disappear, but instead form sieve plates (perforated cross walls).
The strands of cytoplasm can pass through the sieve plates.
Sieve tube elements cannot keep themselves alive and need to be aided by companion cells (like a life support system).
Companion cells and sieve tube elements are linked by plasmodesmata – gaps in cell walls which allow the cytoplasm to link.
Companion cells carry out the metabolic processes (e.g. respiration, excretion) needed to load assimilates into the sieve tubes.
Companion cell adaptations:
Large nucleus
Dense cytoplasm
Many mitochondria to provide ATP for active transport
Many infoldings in their cell surface membranes to give an increased surface area for active transport